The invention relates to a method for adjusting an excitation frequency of an oscillating circuit of a corona ignition device.
Document WO 2010/011838 A1 discloses a corona ignition device with which a fuel/air mixture can be ignited in a combustion chamber of an internal combustion engine by a corona discharge created in the combustion chamber. This corona ignition device comprises an ignition electrode, which is inserted in an insulator. The ignition electrode, the insulator and a sleeve enclosing the insulator form an electrical capacitance. This capacitance is part of an electric oscillating circuit of the corona ignition device, which is excited with a high-frequency alternating voltage of 30 kHz to 50 MHz, for example. The result is a voltage step-up at the ignition electrode, which results in the formation of a corona discharge at said ignition electrode.
In order to ensure efficient operation, it is important that the excitation frequency of the oscillating circuit be as close to the resonant frequency thereof as possible.
Document WO 2010/011838 A1 discloses that the frequency of the oscillating circuit is regulated by measuring the phase shift between current and voltage at the feed points of the oscillating circuit and regulating said phase shift to the value zero by means of a phase-locked loop. In a series oscillating circuit, current and voltage are in phase (phase shift=zero) in the resonant state. The phase-locked loop controls the switching rate of a switching device, by means of which a predefined voltage is applied, in alternation, to one primary winding and to the other primary winding of the transformer. Thus current and voltage are in phase with one another on the secondary side of the transformer, at the feed points of the series oscillating circuit.
In prior art systems and methods, the shift of the resonant frequency of the HF oscillating circuit, which contains the HF ignition device, is a major problem. Different causes are involved here. One cause of the shift of the resonant frequency involves changes in temperature, humidity, contamination of the tip or tips of the ignition electrode of the HF ignition device, and changes in parameters related to the operation of the internal combustion engine. Adjusting the excitation frequency to the resonant frequency by means of a phase-locked loop, as is disclosed in WO 2010/011838 A1, is complex, however, and only partially solves the problem. The reason therefor is that the phase control is susceptible to a temperature drift of the components of the phase-locked loop and to voltage noise.
In order to avoid the disadvantages of a phase-locked loop, it is known from DE 10 2011 052 096 A1 to monitor the instantaneous values of current or voltage of the oscillating circuit and feed the high-frequency generator with primary voltage pulses that are initiated or terminated when the instantaneous value of current or voltage falls below or exceeds, respectively, a predefined switching threshold. This method has the disadvantage of being highly complex in terms of measurement technology.